This invention relates to a Schmitt trigger circuit in which the input threshold voltages associated with the rise or fall of the output voltage have a hysteresis characteristic, that is, the low input threshold voltage (V.sub.L), when the input voltage varies from a high level (H) to a low level (L), is different from the high input threshold voltage (V.sub.H) when the input voltage varies from the L to the H, and more particularly, to a Schmitt trigger circuit in which the input current does not increase suddenly when the input voltage falls from H to L.
A Schmitt trigger circuit, having a high input threshold voltage (V.sub.H) associated with the rise of the input voltage and a low input threshold voltage (V.sub.L) associated with the fall of the input voltage, has the advantages of reduced noise sensitivity and of no oscillation near the point of the threshold voltages due to the hysteresis characteristic of the input threshold voltages, as hereinafter described in detail with respect to conventional Schmitt trigger circuits. Therefore, such a Schmitt trigger circuit is advantageously used as an input buffer for various logic circuits, such as flip-flops, shift registers, counters, etc., because the input of the input buffer usually tends to receive noise and is connected to a long bus line along which the input waveform tends to deteriorate.
However, in a conventional circuit, there is the disadvantage of a sudden increase in the input current when the input voltage falls from the high input threshold voltage (V.sub.H) to the low input threshold voltage (V.sub.L). Because of this sudden increase in the input current, a drive circuit for drawing the input current from the input stage of the Schmitt trigger circuit must have the capability of drawing a larger current than the usual input current.